1. Field of the Invention
The present invention relates to a transparent electrode capable of improving etching characteristics. The present invention is also directed to an etchant suitable for etching a transparent electrode.
2. Description of the Related Art
Generally, a liquid crystal display (LCD) of an active matrix driving system uses thin film transistors (TFT's) as switching devices to display a natural moving picture. Since such a LCD can be made into a smaller device in size than the existent Brown tube, it has been widely used for a monitor for a personal computer or a notebook computer as well as for office automation equipment such as a copy machine, etc. and portable equipment such as a cellular phone, a pager, etc.
Recently, the active matrix LCD trends toward overlapping a pixel electrode 108 of a transparent electrode with a signal wire 106 such as a gate line or a data line as shown in FIG. 1 so as to enhance the aperture ratio. In this case, an organic insulating film 104 having a low dielectric constant is entirely coated on a substrate 102 provided with the signal wire 106 in order to minimize insulation between the signal wire 106 and the pixel electrode and a parasitic capacitance. The LCD having the pixel electrode 108 overlapped with the signal wire 106 has an aperture ratio which is improved by the overlapping area between the signal wire 106 and the pixel electrode 108 in comparison to a LCD in which the pixel electrode 108 does not overlap with the signal wire 106, but rather is spaced, by a desired distance (i.e., about 5 to 10 μm) from the signal wire 106.
The pixel electrode 108 is usually made from a transparent conductive material such as indium-tin-oxide (ITO). This ITO film is entirely deposited on the organic insulating film 104 and thereafter patterned in such a manner to overlap with the signal wire 106. Upon patterning of the ITO film, however, the edge portion of the ITO film may be often twisted or damaged after being etched with an etchant. If the edge portion of the ITO film formed on the organic insulating film 104 is twisted or damaged, then the width of the overlapping portion between the pixel electrode 108 and the signal wire 106 is narrowed which generates light leakage from the overlapping portion. Pattern badness of the ITO film is caused by a poor interface-bonding characteristic of the ITO film to the organic insulating film 104. Particularly, pattern badness is caused by a fact that the ITO film is etched with an etchant which has a high concentration of strong acid and also the etching of the ITO film is made at a low speed.
In order to reduce the bad pattern of the ITO film, a surface treatment process of the organic insulating film 104 for strengthening the adhesive force between the organic insulating film 104 and the ITO film is performed. An example of such a surface treatment includes a method of forming an acid film on the organic insulating film 104 using an acid such as HNO3 or H2SO4, etc. Other examples include a method of forming a hydrogen film on the surface of the organic insulating film 104 or ion-doping the surface of the organic insulating film 104 with oxygen. Even when a surface treatment of the organic insulating film 104 is made, if the ITO film is etched by a doping system in which the substrate 102 is precipitated within a chamber filled with an etchant, then the etchant permeates into the interface between the organic insulating film 104 and the ITO film to generate the pattern badness of the ITO film as mentioned above. In this case, the etchant for etching the ITO film includes a high concentration of strong acid, and the etching thereof is conducted for a long time. For instance, the etchant for etching the ITO film has a very high acid concentration such that the ratio of oxalic acid (C2H2O4) to de-ionized water is less than 1 to 10. The time required for etching the ITO film using such a high-concentration of etchant is more than 1000 seconds.
The ITO film has applications for a display device and an ink-jet head besides a LCD. Also, the ITO film is applicable to a pixel electrode for an X-ray detecting device having a structure similar to the active matrix LCD. However, in the device or equipment employing the ITO film, a transparent electrode or a pixel electrode, etc. applied with ITO is liable to generate a bad pattern because the etching characteristics of the ITO is poor.